Bronze Diffusion Process

NbsSn filament conductors are generally fabricated by means of a solid-state diffusion process ( ). In this the niobium rods are placed into a bronze matrix and jointly extruded. Once the final dimension is attained, the wire is subjected to heat treatment upon which tin selectively diffuses from the copper-tin matrix into niobium. This process is often executed after solenoid winding since the heat-treated wire can only to a limited extent be mechanically deformed. 

The external diffusion process is based upon the fabrication of a Nb-Cu-Ta composite in a manner analogous to the processing used for NbTi. The surface of a wire strand is plated or dip-coated with Sn which is then diffused and reacted to form NbaSn. The cost of this process is lower than that of current bronze processes, since the use of a Cu matrix removes the need for repeated anneals during drawing, and multiple extrusions are not required. Because the amount of Sn is not restricted to 13 wt.% of the matrix, a high overall critical current density can be achieved without the need for thin reaction layers and very fine filaments (--2 pm in diameter). Thus, a single extrusion with several hundred filaments, compared with thousands for the bronze process, is all that is required to obtain a high critical current density in an external diffusion conductor. 

Configurations 3b and 4c were originally devised to allow the diffusion of additional tin from the surface of separately fabricated bronze matrix cores to the filaments by a hybrid bronze and external diffusion process. But current densities... 

Ejectors are available in many materials of construction to suit process requirements. If the gases or vapors are not corrosive, the diffuser is usually constructed of cast iron and the steam nozzle of stainless steel. For more corrosive gases and vapors, many combinations of materials such as bronze, various stainless-steel alloys, and other corrosion-resistant metals, carbon, and glass can be used. 

An example where one metal melts before the densihcation process, is the formation of bronze from a 90 10 weight percentage mixture of copper and tin. The tin melts at a temperature of 505 K, and the liquid immediately wets the copper particles, leaving voids in the compact. The tin then diffuses into the copper particles, leaving further voids due to dre Kirkendall effect. The compact is therefore seen to swell before the hnal sintering temperature of 1080 K is reached. After a period of homogenization dictated by tire criterion above, the alloy shrinks on cooling to leave a net dilatation on alloy formation of about 1%. 

Easy speciation and isotope detection have been identified as playing important roles in optimizing the production of superconducting NbsSn wires by heating a composite of bronze and niobium filaments. Application of TOF-LMMS has provided evidence of the out-diffusion and accumulation of phosphorous, inhibiting Nb3Sn formation when tin diffuses inward. The preferential incorporation of the lower tin isotopes in the NbsSn layer has confirmed the process to be driven by diffusion. The semiconductor industry has used TOF-LMMS to verify the lateral diffusion of dopants, e.g., boron in tantalum and cobalt silicide ruimers only 3 pm wide. Another application is the identification of microscopic residues left on integrated circuits after removal of the photoresist. 

Bronze process The initial rod composite contains Nb rods surrounded by Cu—Sn alloy rods (13-15 wt%Sn) in a suitable arrangement such that after processing into wire form the diffusion-reaction treatment leads to the formation of Nb3Sn at the interface of Nb and Cu—Sn. The initial composite, if composed in a suitable configuration, has excellent deformation properties such that a geo-... 

Bronze process External diffusion Internal diffusion... 

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